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Topic Review (Newest First)

07-30-2013 04:17 PM

rogus

I appreciate all the help. Probably won't get to work on the car till friday. Will update then.

07-29-2013 03:24 PM

cobalt327

Quote:

Originally Posted by rogus

It stumbles under hard acceleration. The spring tension, should it be increased or decreased?

Increased- IF it's being caused by the air valve opening too quick. But it may be the accelerator pump giving an insufficient shot or the power piston spring too weak, or the secondary choke pulloff is too fast, not enough timing or even too much timing, etc.

Start out by setting up the ignition advance first. THEN work on tuning the carb. Info on setting the advance curve here.

Quote:

if it is a timing problem would that normally be too far advanced or retarded?

Most often the initial advance is too low. But you can't just add a bunch of initial timing w/o seeing where that puts the total timing. This is covered in the link above.

Quote:

One last question. I noticed when I rebuilt the engine the vacum measured at the intake manifold increased to 25 inches while at idle (about 1000 rpm). Can this be part of the problem?

This could be caused by too much initial timing, idle speed too high (and too high of an idle speed can start the mechanical advance to come in). Unless you have a radical cam, the idle speed needs to be lower.

07-29-2013 03:11 PM

cobalt327

Quote:

Originally Posted by rogus

can anyone tell me if the quadrajet ever came in a size smaller than 750 cfm? If so how do you tell the cfm rating? i am looking for something around 550 cfm for a recently rebuilt 350 sbc.

Q-jets came in two basic castings, 750 cfm and 800 cfm. The difference is in the size of the primary venturi (shown below). There was a one-year-only Pontiac carb ('71 IIRC) that had a booster ring removed from the primary side that added a bit more flow- at the cost of off idle response. This wasn't used again in production Q-jet carbs.

You can restrict the flow of any Q-jet easily enough. This can be done by adjusting the air valve stop and/or the secondary throttle butterflies to not open fully. That said, you can tailor a Q-jet to work perfectly well on a mild 350 sbc w/o worrying about the cfm. The Q-jet was used on the Chevy V6 4.3L engines in trucks- and most of these carbs had the bigger 800 cfm castings, believe it or not.

If you have the carb number, I can give you a good baseline to work from.

The air valve is opening too fast meaning the tension on the spring should be increased.

Turn the spring adjusting screw another 1/4 turn. Make sure to have the screw driver in the adj. screw when loosening the lock screw.

25" of vacuum at 1000RPM means you have a really mild camshaft.

Have you checked the air fuel ratio indications on the spark plugs?

07-29-2013 12:11 PM

rogus

It stumbles under hard acceleration. The spring tension, should it be increased or decreased?

Yes I installed a new engine to ground wire. I don't remember a brace on the starter when I bought the vehicle.

you have to adjust the spring tension on the air door and/or the the rate of extension of the choke pulloff that is normally connected to the door linkage

I have a Haynes manual on rebuilding quadrajets but it doesn't contain tuning information. Can you suggest a book that does?

if it is a timing problem would that normally be too far advanced or retarded?

One last question. I noticed when I rebuilt the engine the vacum measured at the intake manifold increased to 25 inches while at idle (about 1000 rpm). Can this be part of the problem?

07-28-2013 05:23 PM

lmsport

Does the stumble occur under hard accel or normal driving?
If the secondary air door is opening too fast under hard accel, you get the infamous bog that gave us the name Quadra-bog. To control the opening rate you have to adjust the spring tension on the air door and/or the the rate of extension of the choke pulloff that is normally connected to the door linkage.

07-27-2013 04:55 PM

rogus

poor acceleration

the reason I asked was that although the engine is recently rebuilt it stumbles on acceleration. I was told that it was probably due to a too large of a cfm on the carb.

I assume with a quadrajet this shouldn't be the problem?

07-27-2013 03:50 PM

gearheadslife

might want to study up on how a vacuum 2nd carb works..

07-27-2013 03:22 PM

rogus

quadrajet cfm

can anyone tell me if the quadrajet ever came in a size smaller than 750 cfm? If so how do you tell the cfm rating? i am looking for something around 550 cfm for a recently rebuilt 350 sbc.